In order to study the haemodynamic and metabolic changes following bypass surgery, the regional cerebral blood flow (rCBF), the oxygen extraction fraction (rOEF), the cerebral metabolic rate of oxygen (rCMRO2), and the cerebral blood volume (rCBV) were measured using a positron emission tomograph (PET) on 13 patients who had unilateral internal carotid artery and/or middle cerebral artery occlusion. The patients were divided into two subgroups according to pre-operative rOEF values from the arterial occlusion side: the misery perfusion group, which had high rOEF values (> or = 0.56), and the coupling perfusion group, which had normal rOEF values (0.38-0.48). A post-operative PET study was performed 1-2 months and/or 1-5 years following the surgery. Six of the misery perfusion cases showed a post-operative CBF increase, where an accompanying OEF decreased to its normal level, indicating an attenuated misery perfusion state. The CMRO2 values, however, remained low. The other 7 coupling perfusion cases had an ipsilateral CBF increase in the earlier PET study. We conclude that misery perfusion is attenuated following bypass surgery, although the procedure does not consistently improve oxygen metabolism.